As state and federal regulations set more and more stringent emission standards for stationary sources, there exists an increased need for effective and economical abatement techniques to handle air pollutants. The use of organic industrial solvents is widespread and many chemical, petrochemical and pharmaceutical processes are associated with the generation of large volumes of potentially hazardous organic vapors. Reactors, dryers, centrifuges, storage tanks, mixing vessels, purging, stripping or inert gas blanketing operations are some of the known sources of volatile organic compounds or VOCs.
To conform with air pollution control requirements and to improve the economics of a process, it is clearly beneficial for an operator to recover and reuse such volatile organic compounds. Several solvent recovery techniques are known, with condensation being one such technique. In many instances, one large condensation unit is used to treat the combined emissions from an entire industrial facility.
Shell and tube condensers are extensively used in solvent recovery but involve very high capital costs. Many industrial reactors are routinely equipped with water cooled shell and tube condensers from which condensed solvents can be purified and then recycled or stored for later use. It is often the case, however, that water cooled shell and tube condensers are not effective enough in recovering volatile organic compounds, particularly when the partial pressure of the volatile organic compound in the vapor stream is relatively low. One solution to this problem is to add heat transfer surface area for condensation. This, however, renders the condensation equipment large and cumbersome and leads to significant increases in water consumption. Low temperature refrigerants such as mechanically chilled glycol systems can also be used to improve the performance of shell and tube condensers. Even in this case, the surface area available for condensation must be large and the temperature of such mechanical systems is limited to about -100 degrees F. Such temperatures are simply not low enough to recover highly volatile organic compounds.
Lower temperatures can be generated with cryogenic fluids and the use of liquid nitrogen in the tube side of a shell and tube condenser is known. This practice, however, may result in substantial freezing of the volatile organic compound on the cold surfaces of the shell side. As frozen deposits form, the performance of the condenser deteriorates and a point is soon reached where the equipment must be shut down for defrosting. To simulate a continuous process and reduce down time, it is the conventional practice to run two shell and tube cryogenic condensers in an alternating mode. This however introduces additional capital costs.
Several other techniques which use cryogenic fluids to recover volatile organic compounds have been described.
U.S. Pat. No. 4,769,054 discloses a vapor abatement system that removes the volatile organic vapors from a vent gas stream by solidification. In this approach a solvent which has a much lower melting point than the volatile organic compound is chilled in a coil submerged in liquid nitrogen. Afterwards, the chilled solvent is placed in direct contact with the volatile organic vapor which freezes. This approach is cumbersome since the frozen volatile organic compound becomes contaminated with chilled solvent and thus an additional separation step must be introduced in the process. Furthermore, organic ice forms on the coil submerged in liquid nitrogen, reducing the heat transfer efficiency.
U.S. Pat. Nos. 4,444,016 and 4,551,981 describe multistage setups which include two shell and tube condensers. Liquid nitrogen is used at the third stage where it comes in direct contact with a solvent that it chills. The volatile organic compounds are condensed by direct contact with this chilled solvent and cold nitrogen gas. Besides the fact that this approach does not eliminate shell and tube condensers and their high capital costs, it has additional disadvantages. Liquid nitrogen evaporates on contact with the solvent and the cold nitrogen gas dilutes the vent gas, reducing the partial pressure and degree of saturation of the volatile compounds in the gas stream and lowering its recovery rate. Furthermore, because the nitrogen is injected directly into the broth of the organic solvent, it is contaminated and thus must undergo further treatment before it can be vented to the surrounding atmosphere. Opportunities for its reuse without undergoing purification are also limited. And because high pressure nitrogen is mixed with low pressure vent gas, the workable pressure head of the nitrogen is lost. Thus both purification and recompression steps must be undertaken before the nitrogen can be used elsewhere in the process facility.
In summary, each technique described above suffers from one or more disadvantages. Among these disadvantages are the use of expensive shell and tube condensers, the reduction of heat exchange efficiency due to solvent freezing on heat transfer surfaces, and the contamination of the cryogenic fluid with organic material.